1. Field Of the Invention
The present invention is directed to a method for producing a piezoceramic, and in particular to a method for producing a piezoceramic employing doped lead zirconium titanate (PZT) having a titanium/zirconium ratio which is precisely matched with respect to the morphotropic phase boundary.
2. Description of the Prior Art
It is known that optimum material properties are obtained in the production of a piezoceramic on the basis of doped lead zirconium titanate (PZT) when the titanium/zirconium ratio is exactly matched at least in the range of a few thousandths, with respect to the morphotropic phase boundary, when the titanium and zirconium atoms in the ceramic are uniformly distributed over the grains as well as in the individual grains, and when the sintering temperature for the piezoceramic is as low as possible. The latter condition achieves stable and reproducible conditions by minimizing the evaporation of the lead oxide content from the ceramic during the sintering.
These demands can only be met when extremely fine, chemically uniform and pure-phase PZT powders having a suitable stoichiometry are used for shaping the grain compacts, which can subsequently be sintered at low temperature.
Numerous methods are known for producing PZT powders having the required properties. For example, powders can be produced chemically by co-precipitation, spraying reaction or sol-gel processes. These powders already containing all cations including lead. An optimally low sintering temperature is usually achieved by controlling the fineness of the particles.
Known chemical processes, however, have several disadvantages. These disadvantages include the high manufacturing costs of the powders, the usually small batch sizes, and the fluctuation of the composition from batch-to-batch, a distribution of titanium and zirconium in the powder which is still non-uniform and, most importantly an insufficient reproducibility of the optimum composition of the piezoceramic with respect to the morphotropic phase boundary.
For these reasons, an optimized mixed oxide method is therefore often used for practical a piezoceramic in commercial quantities. In this method, a mixture of lead oxide (PbO) dopant oxides, titanium dioxide (TiO.sub.2) and especially finely particulate zirconium oxide (ZrO.sub.2) is ground, dried and, typically, convened (calcined) into PZT powder at approximately 900.degree. C. Due to the different reaction behavior of TiO.sub.2 and ZrO.sub.2 with PbO, however, PZT powder particles arise having a Ti/Zr ratio from PbTiO.sub.3 through PbZrO.sub.3 which is significantly scattered around a desired value. The powder must therefore be ground and mixed a second time. The homogeneity of the titanium/zirconium ratio which can be achieved by diffusion and grain growth in the sintering of this known powder during compression is limited by the sintering temperature, by the particle size, as well as by the inhomogeneity of the powder. The optimum piezoelectric material values of the sintered ceramic, or the optimum composition of the sintered ceramic with respect to the morphotropic phase boundary, are empirically set for each process on site, and may be possibly re-adjusted by a fine variation of the oxide mixture with respect to the Ti/Zr ratio. A reproducible, although frequently complicated, production of PZT piezoceramics is thus possible by means of these measures.
A combination of the known mixed oxide method and the known chemical processes for producing a PZT ceramic is described in Ceramic Bulletin, Vol. 71, No. 6, (1992), page 978. In this article, the author proposes that ZrTiO.sub.2 (ZTO) having a variable Ti/Zr ratio be produced as a uniform predecessor compound employing a technique known as the hydrothermal method in a two-stage process. The resulting predecessor compound is then calcined and only thereafter is the calcined compound ground with PbO and mixed therewith. In a further calcining step, the lead oxide reacts very easily with the ZTO and forms a pure-phase PZT powder having a uniform Ti/Zr ratio.
A disadvantageous of this method, however, is that the precipitation of the corresponding oxides, which is accomplished chemically, leads to deviations, albeit slight deviations, in the Ti/Zr ratios from batch to batch. Because the composition is usually selected in the proximity of the morphotropic phase boundary, this can result in a large variation in the properties of the piezoceramic thus obtained.